Crosstalk reduction in communication systems



Feb. 16, 1937.

Rt] i, Circuit 1 Patented Feb. 16, 1937 unirti. sir/iras CROSSTALK REDUCTEON IN CGMMUNCA- TION SYSTEMS Harry Nyquist, Millburn, N. J., assigner to American Telephone and Telegraph Company, 'a corporation of New York Application August 27, 1935, Serial No. 38,101

11 Claims. (Cl. 179-78) This invention relates to crosstalk reduction Vector diagram of Fig. 2a in which the direct and more particularly, to crosstalk reduction in transmission current i1 of circuit i is represented a plurality of transmission lines used for com' by the vector i1. Due to the direct transmission rnunication purposes. Still further, it relates to current of iz, a small component of this current, 5 such reduction in a cable of a plurality of coaxial represented by the curved arrow i2 of Fig. l, willl 5 conductors. crosstalk from circuit 2 to circuit I and is shown Its purpose is to reduce or eliminate the crossin Fig. 2a by vector i2. The resultant current talk on a line due to a plurality of other lines by which enters the repeater in this circuit thus bepositively introducing in the line or circuit a comes the Vector sum oi i1 and i2 and the original l component of signal oi appropriate magnitude current i1 has now been changed in magnitude 10 and phase. A further purpose is to reduce such and phase. In order to correct this condition, crosstalk in each and all of a plurality of crossa small part of the output current, indicated by talking lines. -i2', is withdrawn from the repeater in circuit 2 The invention will be better understood by refand is fed back 180 out ci phase and thus exactly l5 erence to the following speciiication and the acneutralizes the current i2' which was originally l5 companying drawing in which Figure l shows a induced from circuit 2 into circuit I. rlhus the scheme for crosstalk reduction in the case of vector diagram of Fig. 2a, shows -iz as 186 out two pairs of adjacent wires and is given for illusof phase with i2 and the resultant current of i1 trative purposes; Figs. 2a and 2h are explanatory in circuit l is unchanged in magnitude and phase. Vector diagrams; Fig. 3 shows my invention as Fig. 2b shows an exactly analogous vector dia-i'` 20 applied to three pairs of transmission lines; Fig. gram for the circuit 2. 4 gives a circuit arrangement for seven pairs of It is apparent that in an open pole line system such lines; and Fig. 5 shows a splicing diagram or in a cable having a great many circuits, either by which the crosstalk exposure for each oi the of the ordinary twisted pair or the coaxial conseven coaxial conductor pairs in the cable of ductor type, there arealarge number of couplings5-25 Fig. 4 is brought to the same value. between any one circuit and all the remainin Referring to Fig. l, there is shown a scheme circuits, the magnitude of the respective couplings for the reduction cf crosstalk in a pair of transdepending upon the exposure of the one circuit mission lines such as may o-ccur in open-wire cirwith respect to the others. In order to carry cuits or in a cable. In this iigure, transmission into eiiect, then, the idea described in connectionf 30 is indicated as being in the same direction for with Fig. l, it is evident that a large number oi' both circuits, the currents being respectively i1 mutual admittances would be required. The and i2. If the line is of considerable length, as situation would be much simplified if one mutual would ordinarily be the case, repeaters would be admittance or mixing circuit could be used for introduced at suitable points such as indicated. all instead oi one for each circuit, and this bem.: 35 In View of their proximity to each other, there comes possible provided the crosstalk from any would be a certain amount of crosstalk from cirone pair to any other has the same magnitude cuit I to circuit 2 and also from circuit to cir irrespective of the combination chosen. It is cuit I, The magnitude and phase oi the crosspossible to accomplish this by splicing successive 40 talk from circuit I to circuit 2 at the input of reel lengths in such a manner that along a re- 40 repeater R. may be indicated by i1' and that from peater section each circuit within the cable has circuit 2 to circuit I by i2', the amount depending a total equal exposure to every other circuit, and upon the degree of coupling between the two cirthat each exposure results in the same amount cuits. If, now, mutual admittances, having propof crosstalk whenever it occurs. This situation er value, are connected between these circuits may be obtained in an open-wire, pole line sys- 45 and current of the appropriate magnitude and tem or in a cable with twisted pairs, although in phase is fed through these admittances, then it general it would be with difficulty. It is more is possible to balance out the crosstalk from each readily obtained in the type of crosstalk which circuit to the other. In the figure, this is indioccurs between parallel coaxial conductors becated by the feeding of power from the output cause of the use of shielding to reduce the cross- 50 of repeater R in circuit i tothe input of repeater talk. This results in a crosstalk which isvery R in circuit 2, and similarly, from the output uniform and symmetrical and does not depend of repeater of circuit 2 to the input of repeater largely upon the geometrical perfection of the of circuit I. arrangement of the conductors which can be This is made more evident by reference to the reached in manufacture but is determined by 55 't iii vii' the primary dimensions and arrangements of these conductors and is substantially independent of any small deviations in them. While my invention is applicable to any of the three types of circuits mentioned, it is peculiarly adapted to coaxial conductors for the reasons given above, and I nd that they will permit the use of one mixing circuit for the purpose of feeding back currents such that they will balance out the crosstalk in the various circuits.

As explanatory of my invention, whether applied to one type of transmission line or another, there is shown in Fig. 3 the currents when three circuits are employed. In this case, if the circuits have been designed or adjusted for equal exposure between repeater stations, then there will be equal components of crosstalk current from circuit I to circuits 2 and 3. Equal cornponents also crosstalk from circuit 2 to circuits I and 3, and a similar condition occurs due to the current in circuit 3. With the proper splicing arrangement, so as to give Vequal exposure, the crosstalk in all three circuits will then have equal magnitudes and in order to compensate for this crosstalk, equal amounts ofV currents are fed through a mixing device A to the'input side of the repeaters in such a manner that they will be out of phase and, consequently, balance out the crosstalk in each circuit. This incidentally results in feeding back a small amount of main transmission in reverse phase but this sirnply reduces the repeater gain somewhat and is,

therefore, of minor importance.

In Fig. 4 there is shown a repeater station at some intermediate point in a cable, thetwo sheath ends of which are shown at s. The cable is indicated as carrying seven conductor pairs although the number may be greater and, in this particular instance, these pairs are coaxial conductors as indicated by means of the circles C. Each coaxial pair as it comes out of one cable section` passes through a repeater and then on into the next cable section. AFromthe output of the repeaters of each of the coaxial pairs numbered I'to 7, there are derived circuits which are connected in parallel to the input of an equalizer E, the circuit from which then continues to a phase corrector H and a repeater R1. From the output of the repeater, the circuit divides with a branch going to the input of each of the repeaters.

On the assumption that, previous to going to the repeater station, the various sections have been so spliced that each one of the circuits I to 1 has the same exposure to each of the other circuits, crosstalk coupling toeach circuit will be identically the same. Under these circumstances, the feed-back of power from the output of each of the repeaters into the mixing repeater R1 and from there to the input of each of the repeaters will bring about the neutralization of crosstalk, all in a manner described in connection with Figs. 1 to 3.

It is, of course, important that there shall be no appreciable reaction directly from the output of one circuit such as 2 to the output of another Y increase in frequency. On the other hand, for the coaxial construction of conductors it decreases with an increase in frequency. In either event, some device is required which will permit the correct amount of current to be fed back at all frequencies and the equalizer E is given such frequency characteristics as to bring this about. Following the equalizer, the currents pass through a phase corrector H, the purpose of which is to allow for the original phase shift in the crosstalk and to correct any phase shift introduced by the equalizer and amplifier so that the total shift in phase with respect to crosstalk will be 180. The necessity of the amplifier R1 depends upon the values which must be assigned to the artificial lines or attenuators and upon the amount of crosstalk existing. It is, however, a desirable device in itself in this connection because of its inherent unilateral properties. From the output of the repeater, which automatically reverses the phase 180, the currents are fed through other artificial lines A2, the purpose of which has already been explained, back to the input side of the various repeaters in direct phase opposition tothe original disturbing currents, thus balancing out the crosstalk.

The matter of obtaining the same exposure for I each transmission line from one repeater station to the next repeater station obviously becomes of importance in the carrying out of this invention.

In the case where seven conductor pairs are used and these are not symmetrically located with respect to each other, it can be readily shown that there would be 2| different coupling coeflicients, and to obtain equalization 2| transpositions or" splicing arrangements will have to be used. `If there is any symmetry, then this number will, of course, be reduced. Thus, in a cable sheath carrying seven coaxial conductors, one centrally located and the other six circularly spaced around this central one, the different kinds of couplings will be reduced to four, indicated by the letters a, b, c and d of Fig. 5. The couplingfrom I to B will be the same as from I to 2 and from I to 5 the same as from I to 3.

In this case,`it can be shown that equality of exposure can be obtained vbetween two points at which the feed-back arrangement is to be applied, is divided into seven sections with transposition at the splicing points as indicatedby the successive rings, there will then be equal crosstalk exposure for each conductor pair from all of the others.

What is claimed is:

l. In a plurality of more than two transmissionV lines each giving crosstalk to the others and containing repeaters, the method of eliminating crosstalk which consists in rendering the exposure of each line to the others the same for each line and supplying to each line at the input of a repeater a definite amount of power from the output of the repeaters of all the other lines of such magnitude and such phase relationship as torneu-- tralize crosstalk therein from the other lines.

The positions of the cir-V 2. In a communication system, a plurality of more than two pairs of transmission lines subject to crosstalk from each other and adjusted so that each line has the same crosstalk exposure to all the other lines, a repeater in each line at a given point, circuits for withdrawing power from the output of each repeater, bringing these circuits to a common mixing circuit and supplying power from the output of the mixing circuit to the input of each repeater of such magnitude and such phase as to neutralize the crosstalk.

3. In a communication system, a plurality of more than two pairs of transmission lines subject to crosstalk from each other and adjusted so that each line has the same crosstalk exposure to all the other lines, means for withdrawing power from each line to a common mixing point, and means for supplying power from the common mixing point from all the lines to each line of such magnitude and such phase relationship as to neutralize the crosstalk.

4. In a communication system, a plurality of more than two pairs of transmission lines subject to crosstalk from each other and adjusted so that each line has the same crosstalk exposure to all the other lines, a mixing circuit, a derived circuit from each line at a given point bringing power from each line to the input of the mixing circuit, a circuit from the output of the mixing circuit to each line and associated therewith to supply power to each line from all the other lines of magnitude and phase to neutralize crosstalk in the lines.

5. In a communication system, a plurality of more than two pairs of transmission lines subject to crosstalk from each other and adjusted so that each line has the same crosstalk exposure to all the other lines, a unilateral repeater, a derived circuit from each line to the input of the repeater acting as a mixing circuit, a circuit from the output of the repeater to each line and associated therewith to supply power to each line from all the other lines of magnitude and phase to neutralize crosstalk in the lines.

6. The combination of claim 4 characterized by the fact that there is a phase corrector in series in the mixing circuit.

7. The combination of claim 4 characterized by the fact that there is a phase corrector and an equalizer in series with the mixing circuit.

8. The combination of claim 4 characterized by the fact that there are attenuating means in each circuit from the lines to the mixing circuit.

9. The combination of claim 4 characterized by the fact that there are attenuating means in each circuit from the lines to the mixing circuit and from the mixing circuit to the lines.

l0. The combination of claim 5 characterized by the fact that there are attenuating means in each derived circuit from the lines to the mixing circuit and from the mixing circuit to the lines.

11. The combination of claim 5 characterized by the fact that there are attenuating means in each circuit from the lines to the mixing circuit and from the mixing circuit to the lines and that the mixing circuit itself includes a phase corrector and an equalizer.

HARRY NYQUIST. 

